On the screen, everything you type looks good until you print it out. Some think all the output of a calculator is correct.
Likewise they are happy to be corrected wrongly by Spell Chuckers. This is effective because of

a) laziness and

b) it looks good on the screen

Here is an exercise:

The same particular errors creep into assignment work. Get used to printing out and editing your own work with a pen.

To practice this, correct the following passage:

Lecturer require all work to be handed on time. Students work must be checked thoroghly for spelling Punctuation and grammar, students must word process there assignments, the spell checker on ms word will not pick errrors like using the wrong world. You should read out your work to yourself or perhaps you could have asked someone else, to read it to see if it all made sense

Print it out and try it.

Then go through my answer which can be found
here.